In U.S. Pat. No. 3,939,354, assigned to the same assignee as the present application and incorporated herein by reference, there is disclosed a technique for isotope separation, particularly uranium enrichment, in which a uranium vapor flow is selectively photoexcited and ionized, resulting in the generation of a plasma. Ions in the plasma are periodically accelerated towards collecting surfaces by an electrode structure typically comprising a set of adjacent U-shaped channels forming one electrode, and a set of elongate electrodes running through each channel, forming another set of electrodes. It is suggested there that a source for electrons coming other than from the vapor source, may be desired for efficient use of the ion acceleration structure, and means in the nature of a filament are provided for furnishing electrons to the plasma. To supply all of the electrons required for the extraction system from a filament of the type illustrated in the above-identified patent, for example approximately 1 ampere per square centimeter of cross-sectional flow area within the extraction region, using conventional filamentary electron sources, requires a very heavy power consumption and large surface area, particularly when practiced on a production scale.